1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to molded case circuit breakers having a magnetic trip unit including an armature and a magnetic coil having a plurality of turns that occupies the same volume as a single turn magnetic coil.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well-known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are generally used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and a relatively high level short circuit condition. An overload condition is normally 200 to 300 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers generally include at least one pair of separable main contacts which may be operated manually by way of an operating handle, extending outwardly from the circuit breaker case, or automatically in response to an overload or a high level short circuit condition. In the manual mode and one automatic mode of operation, the separable main contacts are opened by an operating mechanism which, in turn, is actuated by either a trip unit in the automatic mode or the operating handle in the manual mode. In another automatic mode of operation, magnetic repulsion forces, generated between the stationary and movable main contacts during relatively high level overcurrent conditions, can also cause the main contacts to be separated independently of the operating mechanism.
In the first mentioned automatic mode of operation, trip units, which may be combination thermal magnetic trip units or magnetic only trip units are used to sense an overcurrent condition. These trip units are interlocked with the circuit breaker operating mechanism to cause the separable main contacts to be tripped during an overcurrent condition.
Various types of trip units are known. In one type, the trip unit is formed as a part of the circuit breaker. In another type, the trip unit is interchangeable and is formed as a modular unit with a separate housing which can easily be inserted and removed from the circuit breaker as a unit.
Depending upon the type of electrical load to be protected, some trip units are provided with magnetic only trip units, such as in the case of electric arc welders. In other applications, a magnetic only trip unit is provided on the circuit breaker when the overload protection is provided in another device, such as a motor contactor, generally provided with its own overload relays. However, for many electrical loads, such as electrical motors, both overload and short circuit protection is required by the circuit breaker Accordingly, combination thermal magnetic trip units, which may be adjustable, are provided on circuit breakers supplying such electrical loads. Examples of such adjustable thermal magnetic tripping units are disclosed in U.S. Pat. Nos. 4,691,182; 4,698,606 and 4,725,800, all assigned to the same assignee as the present invention and hereby incorporated by reference.
The combination thermal magnetic trip units are comprised of a thermal unit and a magnetic unit. The thermal unit consists of one or more bimetals which cause the circuit breaker to be tripped during an overload condition. The bimetals are generally disposed in series with a line conductor and thus are subjected to line current. During normal operating conditions, the bimetals are deflected by the line current flowing therethrough but not enough to cause a trip of the circuit breaker. During an overload condition, the bimetals are subjected to additional heat resulting from the increased current flow therethrough which causes additional deflection of the bimetals resulting in tripping of the circuit breaker
The magnetic unit includes a magnetic coil assembly and a pivotally mounted armature. The pivotally mounted armature is generally interlocked to trip the circuit breaker during relatively high level overcurrent conditions, such as a short circuit condition. Due to space restrictions in smaller frame size molded case circuit breakers, a single turn magnetic coil, commonly formed from a generally U-shaped conductor, is disposed in series with the line conductor and disposed about a magnetic core forming a magnetic coil assembly. The pivotally mounted armature is disposed at a predetermined air gap from the magnetic coil assembly. When the electrical current through the U-shaped conductor becomes relatively high, such as during a short circuit condition, sufficient magnetic attraction forces are generated in the magnetic coil assembly to attract the armature. Since the armature is interlocked with the operating mechanism, this action causes the circuit breaker to be tripped.
The tripping force of the magnetic trip unit is generally a function of the product of the electrical current therethrough and the number of turns provided in the magnetic coil assembly. The magnetic tripping force is generally set to trip at 15 times the continuous electrical current rating of the circuit breaker Thus, for a 150 amp frame circuit breaker, normally used in applications where the desired continuous electrical current of the circuit breaker is between 15 amperes and 150 amperes, the magnetic tripping force would thus be set to trip at between, for example, 225 amperes or 2,250 amperes, respectively, such as in a Westinghouse type HFD molded case circuit breaker.
In such molded case circuit breakers, since a single turn magnetic coil is generally provided due to space limitations, the tripping force thus becomes proportional to the electrical current through the single turn magnetic coil.
However, there are instances where it is desirable for the magnetic trip unit to trip the circuit breaker at magnitudes far less than 15 times the full load rating of the circuit breaker. For example, there are certain applications where it is desirable to trip at one-half of the normal value (e.g., 110 amperes for a 15 ampere circuit breaker).
There are several ways to adjust the trip rating of the magnetic trip unit. One way is to adjust the air gap between the armature and the magnetic coil assembly. Specifically, in order to lower the trip rating of the magnetic trip unit, it would be necessary to reduce the air gap between the armature and the magnetic coil. However, due to the environmental factors, especially vibrations, that many circuit breakers can be subjected to, it is undesirable to reduce the air gap since this may cause spurious trips of the circuit breaker.
Another way to reduce the electrical current at which the magnetic unit trips the circuit breaker, is to increase the tripping force of the armature assembly. The tripping force of the armature assembly may be increased by increasing the number of ampere turns in the magnetic coil. However, due to space constraints within many known molded case circuit breakers, increasing the ampere turns in the magnetic coil in the magnetic trip unit requires additional volume which generally requires a larger frame size to be used. However, using such a larger frame size for molded case circuit breakers having relatively lower magnetic trip ratings greatly increases the cost of such circuit breakers. Additionally, it frustrates the desire to standardize among molded case circuit breakers having various magnetic trip ratings.
In some known circuit breakers, such as in a Westinghouse type HMCP, additional windings are provided the magnetic coil. However, to accommodate the additional space requirements for such additional windings, the type HMCP is provided as a magnetic-only circuit breaker. In other words, such a circuit breaker is provided without a thermal unit to accommodate the additional space required for the additional windings in the magnetic unit. Thus, in such an application, if thermal protection is also required, it must be provided as a separate unit, such as a thermal overload relay. Such an application would result in substantially increased cost to the end user.